Mechanical locking differential lockout mechanism

ABSTRACT

An improved differential gear mechanism is characterized by a lockout mechanism ( 63 ) operably associated with a flyweight mechanism ( 53 ) and including a lockout member ( 65, 101 ) positionable, in response to an input signal, in a normal condition and a lockout condition. In the normal condition, the lockout member ( 65, 101 ) permits a flyweight stop surface ( 57 ) to move from the retracted position to the extended position. In the lockout condition, the lockout member ( 65, 101 ) prevents the flyweight stop surface ( 57 ) from moving from the retracted position to the extended position.

BACKGROUND OF THE DISCLOSURE

The present invention relates to differential gear mechanisms, and moreparticularly, to such mechanisms of the type commonly referred to as“locking differentials”. More specifically, the present inventionrelates to mechanisms of the type also referred to as “mechanicallockers”, i.e., locking differentials in which the locking functionoccurs in response to the operation of a mechanical device, as opposedto hydraulic actuation or electromagnetic actuation.

A conventional locking differential made by the assignee of the presentinvention utilizes a flyweight mechanism to initiate the lock-up of thedifferential clutch, wherein the flyweight mechanism then retardsrotation of the cam plate relative to the differential input (i.e., thering gear and differential case). Locking differentials of the type thatutilize a flyweight mechanism to initiate clutch engagement are now wellknown, and may be made in accordance with the teachings of any one ormore of U.S. Pat. Nos. 3,606,803; 5,484,347, and 6,319,166, all of whichare assigned to the assignee of the present invention and incorporatedherein by reference. However, it should be understood that the presentinvention is not limited to only locking differentials that are made inaccordance with the teachings of the cited patents.

The locking differentials of the type made and sold commercially by theassignee of the present invention have been in widespread commercialusage for many years, and have performed in an extremely satisfactorymanner, especially on vehicles which operate, for at least part of theirduty cycle, on rough, uneven terrain, or under conditions of poortraction. Such locking differentials perform especially well when thevehicle is operating on what is referred to as a “split-μ” surface,i.e., wherein the drive wheel on one side of the vehicle has fairly goodtraction, and the drive wheel on the other side of the vehicle has verypoor traction. In such operating conditions, the locking differentialwill engage the locking clutch, thus driving both drive wheels at theinput speed to the differential case (i.e., the speed of rotation of theinput ring gear).

One situation that has been observed, in which the conventional lockingdifferential does not perform in a satisfactory manner, is when avehicle equipped with a mechanical locking differential is operating ina condition in which “spin-out” of one of the driving wheels isinherent. For example, if the vehicle is operating with a mini-spare(i.e., a spare tire which, for purposes of saving space in the trunk, issubstantially smaller than the “normal” tires), the difference in tirediameter will cause operation of the differential flyweight mechanism,and lock-up of the differential clutch pack, even as the vehicle isoperating in a “straight-ahead” mode.

BRIEF SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide animproved differential gear mechanism, of the “mechanical locker” type,in which it is possible to, in some manner, control operation of themechanical device that normally initiates engagement of the differentialclutch, wherein such control is not dependent on only the normal mode ofoperation of the mechanical device.

It is a more specific object of the present invention to provide such animproved differential gear mechanism in which it is possible to“lock-out” or to prevent the operation of, a flyweight mechanism whichreacts to the speed difference between the two differential side gearsand initiates engagement of the differential clutch.

The above and other objects of the invention are accomplished by theprovision of an improved differential gear mechanism comprising a gearcase defining a gear chamber, a differential gear set disposed in thegear chamber, and including at least one input gear and a pair of outputgears defining an axis of rotation. A lock-up clutch is operable toretard differentiating action, and included is an actuating means foractuating the lock-up clutch. The lock-up clutch is operable between anengaged condition, effective to retard relative rotation between thegear case and the output gears, and a disengaged condition. Theactuating means includes cam means operable to affect the engagedcondition of the lock-up clutch, and retarding means operable to engagethe cam means and retard rotation of one member of the cam means. Theretarding means comprises a flyweight mechanism rotatable about an axisoriented generally parallel to the axis of rotation of the differentialgear mechanism, the flyweight mechanism being rotatable at a speedgenerally representative of the extent of the differentiating action anddefining a stop surface moveable from a retracted position to anextended position in response to a predetermined extent ofdifferentiating action. The actuating means further includes a latchsurface disposed to engage the stop surface when the stop surface is inthe extended position.

The improved differential gear mechanism is characterized by a lockoutmechanism operably associated with the flyweight mechanism and includinga lockout member positionable, in response to an input signal, between anormal condition and a lockout condition. In the normal condition, thelockout member permits the stop surface to move from the retractedposition to the extended position. In the lockout condition, the lockoutmember prevents the stop member from moving from the retracted positionto the extended position.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an axial cross-section of a locking differential mechanism(“Prior Art”) of the type with which the present invention may beutilized.

FIG. 2 is a somewhat fragmentary, transverse cross-section of thelocking differential mechanism shown in FIG. 1, and on approximately thesame scale.

FIG. 3 is an axial cross-section, similar to FIG. 1, but on a largerscale, illustrating in somewhat greater detail the flyweight mechanismwhich comprises an important part of the locking differential mechanismof FIG. 1.

FIG. 4 is a perspective view of a locking differential mechanismaccording to an embodiment of the present invention.

FIG. 5 is a detail view of the flyweight mechanism and a lockoutmechanism, which comprise an important part of the locking differentialmechanism of FIG. 4.

FIG. 6 is a partial cross-sectional view of the locking differentialmechanism of FIG. 4, showing a lockout member in a normal condition.

FIG. 7 is a partial cross-sectional view of the locking differentialmechanism of FIG. 4, showing a lockout member in a lockout condition.

FIG. 8 is a perspective view illustrating a locking differentialmechanism according to an alternative embodiment of the presentinvention.

FIG. 9 is a detail view of the flyweight mechanism and a lockoutmechanism, which comprise an important part of the locking differentialmechanism of FIG. 8.

FIG. 10 is a cross-sectional view of a portion of the lockout mechanismshown in FIG. 9.

FIG. 11 is a partial cross-sectional view of the flyweight mechanism andthe lockout mechanism shown in FIG. 9.

FIG. 12 is a partial cross-sectional view of the locking differentialmechanism of FIG. 8, showing a lockout member in a normal condition.

FIG. 13 is a partial cross-sectional view of the locking differentialmechanism of FIG. 8, showing a lockout member in a lockout condition.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawings, which are not intended to limit theinvention, FIG. 1 is an axial cross-section of a locking differentialgear mechanism of the type that may advantageously utilize the presentinvention. The overall construction and operation of the lockingdifferential shown in FIG. 1 is already well known to those skilled inthe art, and is illustrated and described in greater detail in theabove-incorporated patents. The differential gear mechanism as shown inFIG. 1 (“Prior Art”) includes a gear case 11 that defines therein a gearchamber, generally designated 13. Torque input to the lockingdifferential is typically by means of an input gear 15 (shown only infragmentary view in FIG. 1). The input gear 15 (also referred to as a“ring gear”) is intended to be in toothed engagement with an inputpinion gear (not shown in FIG. 1), which receives input drive torquefrom the vehicle driveline. The input gear 15 may be attached to thegear case 11 by means of a plurality of bolts 17.

Disposed within the gear chamber 13 is a differential gear set includinga plurality of pinion gears 19 (only one of which is shown in FIG. 1),rotatably mounted on a pinion shaft 21 (only a portion of which is shownin FIG. 1). The pinion shaft 21 is secured to the gear case 11 by anysuitable means, not shown herein. The pinion gears comprise the inputgears of the differential gear set, and are in meshing engagement with apair of side gears 23 and 25, which comprise the output gears of thedifferential gear set. The side gears 23 and 25 are in splinedengagement with a pair of axle shafts 27 and 29, respectively. The gearcase 11 includes annular hub portions 31 and 33, surrounding the axleshafts 27 and 29, respectively. Typically, bearing sets (not shown) aremounted on the hub portions 31 and 33 to provide rotational support forthe differential gear mechanism, relative to the main, outerdifferential housing (also not shown herein).

During normal, straight-ahead operation of the vehicle, nodifferentiating action occurs between the left and right axle shafts 27and 29, and the pinion gears 19 do not rotate relative to the pinionshaft 21. Therefore, the gear case 11, the pinion gears 19, the sidegears 23 and 25, and the axle shafts 27 and 29 all rotate about an axisof rotation (A) of the axle shafts 27 and 29, as a solid unit.

Under certain operating conditions, such as when the vehicle is turning,or there is a slight difference in the size of the tires associated withthe axle shafts 27 and 29, it is permissible for a certain amount ofdifferentiating action to occur between the side gears 23 and 25, up toa predetermined level of speed difference. Above that predeterminedlevel (e.g., above a difference of about 100 rpm between the side gears23 and 25), indicating that a wheel spin-out is imminent, it isdesirable to retard the relative rotation between each of the side gears23 and 25 and the gear case 11, to prevent excessive differentiatingaction between the axle shafts 27 and 29.

In order to retard differentiating action, the differential gear meansis provided with a lockup means for locking up the differential gearset, and an actuating means for actuating the lockup means. The generalconstruction and operation of the lockup means and the actuating meansare now well known in the art, and will be described only brieflyherein. For a more detailed explanation of the lockup means and theactuating means, reference should be made to the above-incorporatedpatents, and further, to U.S. Pat. No. RE 28,004 and U.S. Pat. No.3,831,462, both of which are assigned to the assignee of the presentinvention.

In the subject embodiment, the lockup means comprises a clutch pack,generally designated 35. As is now well known to those skilled in theart, the clutch pack 35 includes a plurality of outer clutch diskssplined to the gear case 11, and a plurality of inner clutch diskssplined to the side gear 23. Referring still to FIG. 1, the lock-upmeans further includes a cam mechanism, generally designated 41. As iswell known to those skilled in the locking differential art, the primaryfunction of the cam mechanism 41 is to effect movement of the clutchpack 35 from the disengaged condition, as shown in FIG. 1, to anengaged, “loaded” condition (not specifically illustrated herein). Inthe engaged condition, the clutch pack 35 is effective to retardrelative rotation between the gear case 11 and the side gear 23, thusretarding and minimizing differentiating action between the side gears23 and 25.

The cam mechanism 41 includes the side gear 23 and a main cam member 43.The side gear 23 defines a cam surface 45, and the cam member 43 definesa cam surface 47. The cam member 43 also defines a set of external teeth49, the function of which will be described subsequently. During normal,straight-ahead operation of the vehicle, with little or nodifferentiating action occurring, the cam surfaces 45 and 47 remain inthe neutral position shown in FIG. 1, with the cam member 43 rotatingwith the side gear 23, at the same rotational speed. Movement of theclutch pack 35 to the engaged condition is accomplished by retardingrotation of the cam member 43, relative to the side gear 23, to cause“ramping” of the cam surfaces 45 and 47, as is well known to thoseskilled in the cam art. Such ramping results in axial movement of thecam member 43, to the left in FIG. 1, thus initiating engagement of theclutch pack 35.

In order to retard rotation of the cam member 43 relative to the sidegear 23, the locking differential gear mechanism includes a retardingmechanism, generally designated 51, which comprises the actuating meansfor actuating the lockup means. It should become apparent to thoseskilled in the art that within the scope of the present invention, manydifferent configurations and types of retarding mechanisms may beutilized. In the subject embodiment, and by way of example only, theretarding mechanism 51 is of the flyweight type, illustrated anddescribed in greater detail in the above-incorporated patents and hereinbelow. The retarding mechanism 51 is mounted within the gear case 11 forrotation about its own axis, and includes a cylindrical flyweightportion 53. The retarding mechanism 51 further includes an externallygeared portion 55, which is in engagement with the external gear teeth49 of the cam member 43.

Flyweight portion 53 is rotatable about an axis (a), shown in FIG. 5,and oriented generally parallel to the axis of rotation (A), at a speedgenerally representative of the extent of the differentiating action.Flyweight portion 53 includes a pair of flyweight members 56 eachdefining a stop surface 57. The stop surface 57 is moveable from aretracted position (FIG. 2) to an extended position (not shown) inresponse to a predetermined extent of differentiating action. Theflyweight member also defines a pivot portion 59 defining a pivot axisgenerally parallel to and spaced apart from the axis (a) of theflyweight portion 53. The stop surface 57 is generally oppositelydisposed from the pivot axis. The actuating means includes a latchsurface 61 positioned to engage the stop surface 57 when the stopsurface is in the extended position.

During operation, if differentiating action begins to occur between theaxle shafts 27 and 29, the sidegear 23 and cam member 43 will begin torotate in unison at a speed different than that of the gear case 11,causing the retarding mechanism 51 to begin to rotate about its axis (a)at a rotational speed which is a function of the extent of thedifferentiating action. As the speed of rotation of the retardingmechanism 51 increases, centrifugal force causes the flyweights 56 tomove outward until one of the flyweights stop surface 57 engages thelatch surface 61, preventing further rotation of the retarding mechanism51. When the retarding mechanism 51 stops rotating, the engagement ofthe geared portion 55 and the gear teeth 49 causes the cam member 43 torotate at the same speed as the gear case 11 (which is different thanthe speed of rotation of the sidegear 23), resulting in ramping, andinitializing of engagement of the clutch pack 35.

Referring to FIGS. 4-7, a differential gear mechanism is shown that issubstantially similar to the mechanism shown in FIG. 1 with at least oneexception, namely, a lockout mechanism 63 is operably associated withthe retarding mechanism 51. The lockout mechanism 63 allows theretarding mechanism 51 to be selectively deactivated when automaticlocking of the differential is not desired (e.g., when the vehicle isoperating with a mini-spare).

Lockout mechanism 63 includes a lockout member 65 that is positionable,in response to an input signal, between a normal condition (FIG. 6) anda lockout condition (FIG. 7). In the normal condition, the lockoutmember 65 permits the stop surface 57 to move from the retractedposition to the extended position. In the lockout condition, the lockoutmember 65 prevents the stop member 57 from moving from the retractedposition to the extended position.

In an embodiment illustrated in FIGS. 4-7, the lockout member 65 isgenerally cylindrical, having a cup-shaped body moveably supported onthe retarding mechanism 51 between the externally geared portion 55 andthe flyweights 56. In the lockout condition (FIG. 7), the lockout member65 at least partially surrounds the flyweight mechanism 53 to preventthe flyweight members 56 from pivoting outward as the flyweightmechanism rotates. In the normal condition (FIG. 6), the lockout member65 is retracted toward the externally geared portion 55 leaving theflyweights 56 free to pivot outward.

The lockout mechanism 63 also comprises a first, generally annularmember 69 having an electromagnetic coil 71 that may be energized toproduce a magnetic field. A second, generally annular member 73 of knownmagnetic properties is positioned around the first annular member 69 andincludes a ramp 75. First annular member 69 is stationary with respectto the gear case 11, such that the gear case 11 rotates relative to thefirst annular member 69 during operation. The second annular member 73is free to rotate with the gear case 11 when the coil 71 is de-energizedand is inhibited from rotating with the gear case 11 when the coil 71 isenergized by virtue of the second annular member's interaction with themagnetic field generated by the coil. The coil 71 is energized by anelectrical input signal, which is selectively transmitted to the coil 71through an electrical connector 77. The connector is provided incommunication with a controller (not shown), such as the vehicleelectronic control unit (ECU), which controls communication of theelectrical input signal to coil 71.

Referring still to FIGS. 4-7, lockout mechanism 63 also includes amovable, elongated pin 79 connected to the lockout member 65 and adaptedto engage the ramp 75 when the coil 71 is energized to move the lockoutmember 65 toward the lockout condition. The pin 79 is supported by thegear case 11 for movement along an axis generally parallel to axis (a).The pin 79 is laterally offset from retarding mechanism 51 and connectedto the lockout member 65 by a connecting member 81. The pin 79 extendsthrough and exits the gear case 11 adjacent the second annular member73, and includes an end portion 83 orthogonally positioned with respectto a portion of the pin 79 supported within the gear case 11. The endportion 83 engages the ramp 75 when the coil 71 is energized androtation of the second annular member 73 is inhibited. As the endportion 83 travels up the ramp 75 during engagement, the pin 79 is drawnout of the gear case 11 (to the right in FIG. 5) and the lockout member65 is moved toward the lockout condition. When the lockout member 65 isin the lockout condition and the pin 79 is fully extended (FIGS. 5 and7), the second annular member 73 is forced to rotate with the gear case11 and the pin 79 against the anti-rotative magnetic force created bythe coil 71. The ramp 75 includes first and second ramp portions 85 aand 85 b, either of which are engageable by the end portion 83 of thepin 79 depending on the direction of rotation of the gear case 11.

The pin 79 and the lockout member 65 are urged toward the normalcondition by a compression spring 87 positioned between a window surfacedefined by the gear case 11 and a flange 89 on the pin 79. As the endportion 83 moves away from the gear case 11 due to engagement with theramp 75, the spring 87 is compressed. When the coil 71 is de-energizedand rotation of the second annular member 73 is uninhibited, the spring87 will force the pin 79 and the lockout member 65 toward the normalcondition as the end portion 83 moves down the ramp 75.

Referring to FIGS. 8-13, another embodiment of the present invention isshown. In the illustrated embodiment, the lockout mechanism 90 comprisesa two-piece shaft having a first shaft portion 91 and a second shaftportion 93 upon which flyweight mechanism 53 is supported for rotationtherewith. The first and second shaft portions 91, 93 are rotatableabout the axis (a) oriented generally parallel to the axis of rotation(A), at a speed generally representative of the extent of thedifferentiating action. The first shaft portion 91 includes theexternally geared portion 55 that meshes with the external teeth 49 ofthe cam member 43. A first end 95 of first shaft portion 91 is supportedby the gear case 11 in the conventional manner and a second end 97 ofthe first shaft portion 91 is supported by the second shaft portion 93.A first compression spring 99 is disposed between the first and secondportions 91, 93 to urge the first shaft portion 91 away from the secondshaft portion 93.

The first shaft portion 91 includes a lockout member 101 adapted to bereceived in a receptacle 103 of the second shaft portion 93. Lockoutmember 101 comprises a generally polygonal-shaped (when viewed incross-section) protrusion that mates with the correspondingly-shapedreceptacle 103 when properly oriented. In the normal condition, thelockout member 101 is received in the receptacle 103, locking the firstand second shaft portions 91, 93 together for common rotation andpermitting the stop surface 57 to move from the retracted position tothe extended position. In the lockout condition (FIGS. 9-11), thelockout member 101 is not positioned in the receptacle 103, allowing thefirst shaft portion 91 to freely rotate relative to the second shaftportion 93 and preventing the stop member 57 from moving from theretracted position to the extended position. The lockout member 101 andreceptacle 103 configuration illustrated in FIGS. 9-12 is provided byway of example only. It will be appreciated that many different lockoutmember and receptacle configurations may be employed, including, forexample, those configurations where lockout member comprises a differentpolygonal-shaped protrusion (e.g., hexagon, octagon, etc) or a spline,and those configurations where the lockout member 101 is provided on thesecond shaft portion for receipt in a receptacle in the first shaftportion.

Referring still to FIGS. 8-13, lockout mechanism 90 also includes anelectric solenoid 105 attached to the second shaft portion 93 andsupported on the gear case 11 for rotation therewith. The solenoid 105is selectively operable in response to an electric input signal to movethe second shaft portion 93 axially between a position in which thelockout member 101 is in the normal condition (i.e., received inreceptacle 103) and a position in which the lockout member 101 is in thelockout condition (i.e., withdrawn from receptacle 103). An electricslip-ring mechanism 107 may be used to transfer the electric inputsignal from a remote source (e.g., vehicle ECU) to the solenoid 105. Theslip-ring mechanism 107 includes a first annular member 109 that isstationary relative to the gear case 11 and a second annular member 111that is free to rotate with the gear case 11 and the solenoid 105. Thesecond annular member 111 is electrically connected to the solenoid 105by one or more wires 113, and contacts the first annular member 109during rotation to permit the electrical input signal to pass from thefirst annular member 109, through the second annular member 111, andinto the solenoid 105.

The second shaft portion 93 is urged toward the first shaft portion 91by a second compression spring 115 positioned between the solenoid 105and a flange 117 on the second shaft portion 93. When the solenoid 105moves the second shaft portion 93 away from the first shaft portion 91,and the lockout member 101 is in the lockout condition, the spring 115is compressed. When the solenoid is de-energized, the spring 115 willforce the second shaft portion 93 toward the first shaft portion 91,returning the lockout member 101 to the normal condition. The springforce generated by the second compression spring 115 is greater than thespring force generated by the first compression spring 99 to permitreturn to the normal condition.

The invention has been described in great detail in the foregoingspecification, and it is believed that various alterations andmodifications of the invention will become apparent to those skilled inthe art from a reading and understanding of the specification. It isintended that all such alterations and modifications are included in theinvention, insofar as they come within the scope of the appended claims.

1. A differential gear mechanism comprising a gear case defining a gearchamber, a differential gear set disposed in said gear chamber, andincluding at least one input gear and a pair of output gears defining anaxis of rotation; a lock-up clutch operable to retard differentiatingaction, and actuating means for actuating said lock-up clutch; saidlock-up clutch being operable between an engaged condition, effective toretard relative rotation between said gear case and said output gears,and a disengaged condition; said actuating means including cam meansoperable to effect said engaged condition of said lock-up clutch, andretarding means operable to engage said cam means and retard rotation ofone member of said cam means, said retarding means comprising aflyweight mechanism rotatable about an axis oriented generally parallelto said axis of rotation, at a speed generally representative of theextent of said differentiating action, and defining a stop surfacemoveable from a retracted position to an extended position in responseto a predetermined extent of differentiating action; said actuatingmeans further including a latch surface disposed to engage said stopsurface when said stop surface is in said extended position;characterized by: (a) a lockout mechanism operably associated with saidflyweight mechanism and including a lockout member positionable, inresponse to an input signal, in a normal condition and a lockoutcondition; (b) in said normal condition, said lockout member permitssaid stop surface to move from said retracted position to said extendedposition; and (c) in said lockout condition, said lockout memberprevents said stop surface from moving from said retracted position tosaid extended position.
 2. A differential gear mechanism as claimed inclaim 1, characterized by said flyweight mechanism including a flyweightmember defining said stop surface, said flyweight member defining apivot portion defining a pivot axis parallel to and spaced apart fromsaid axis of said flyweight mechanism, said stop surface being generallyoppositely disposed from said pivot axis.
 3. A differential gearmechanism as claimed in claim 1, characterized by said lockout memberbeing generally cylindrical, said normal condition comprising saidcylindrical lockout member being disposed axially adjacent saidflyweight mechanism, and said lockout condition comprising saidcylindrical lockout member at least partially surrounding said flyweightmechanism.
 4. A differential gear mechanism as claimed in claim 1,characterized by said lockout mechanism comprises a first annular memberhaving an electromagnetic coil, a second annular member of knownmagnetic properties positioned around said first annular member andincluding a ramp, and a movable pin connected to said lockout member andadapted to engage said ramp when said coil is energized to move saidlockout member toward said lockout condition.
 5. A differential gearmechanism as claimed in claim 4, characterized by said first annularmember being stationary with respect to said gear case, such that saidgear case rotates relative to said first annular member.
 6. Adifferential gear mechanism as claimed in claim 4, characterized by saidsecond annular member being free to rotate with said gear case when saidcoil is de-energized and is inhibited from rotating with said gear casewhen said coil is energized.
 7. A differential gear mechanism as claimedin claim 4, characterized by said coil being energized by said inputsignal.
 8. A differential gear mechanism as claimed in claim 4,characterized by said pin being supported by said gear case for movementalong an axis generally parallel to said axis.
 9. A differential gearmechanism as claimed in claim 4, characterized by said pin beinglaterally offset from said flyweight mechanism and connected to saidlockout member by a connecting member.
 10. A differential gear mechanismas claimed in claim 4, characterized by said pin extending through andexiting said gear case adjacent said second annular member, said pinincluding an end portion orthogonally positioned with respect to aportion of the pin supported within said gear case, said end portionengages said ramp when said coil is energized and rotation of saidsecond annular member is inhibited.
 11. A differential gear mechanism asclaimed in claim 4, characterized by said ramp including first andsecond ramp portions and, either of which are engageable by said pindepending on the direction of rotation of said gear case and said pin.12. A differential gear mechanism as claimed in claim 4, characterizedby said pin and said lockout member being urged toward the normalcondition by a spring.
 13. A differential gear mechanism as claimed inclaim 1, characterized by said lockout mechanism comprises a two-pieceshaft having a first shaft portion and a second shaft portion upon whichsaid flyweight mechanism is supported for rotation therewith, a firstend of first shaft portion is supported by said gear case and a secondend of said first shaft portion is supported by said second shaftportion, said first shaft portion or said second shaft portion includingsaid lockout member, which is adapted to be received in a receptacle ofthe other of said first shaft portion and said second shaft portion. 14.A differential gear mechanism as claimed in claim 13, characterized bysaid lockout member comprises a protrusion that mates with acorrespondingly-shaped receptacle, whereby in the normal condition, saidlockout member is received in said receptacle, locking said first andsecond shaft portions together for concurrent rotation and permittingsaid stop surface to move from said retracted position to said extendedposition, and in said lockout condition, said lockout member iswithdrawn from said receptacle, allowing said first shaft portion tofreely rotate relative to said second shaft portion and preventing saidstop surface from moving from said retracted position to said extendedposition.
 15. A differential gear mechanism as claimed in claim 13,characterized by said lockout mechanism including a compression springdisposed between said first and second shaft portions and to urge saidfirst shaft portion away from said second shaft portion.
 16. Adifferential gear mechanism as claimed in claim 13, characterized bysaid lockout mechanism further comprising an electric solenoid operablyassociated with said second shaft portion and supported on said gearcase for rotation therewith, and a compression spring that urges thesecond shaft portion toward the first shaft portion, whereby when saidsolenoid moves said second shaft portion away from said first shaftportion and said lockout member is in said lockout condition said springis compressed, and when said solenoid is de-energized, said springforces said second shaft portion toward said first shaft portion,returning said lockout member to said normal condition.